1. Technical Field
The present invention relates to a paper drying machine and a method of drying a paper web in a paper drying machine. More particularly, the invention pertains to a paper drying machine which employs infrared drying of a paper web.
2. Background Art
Paper products have conventionally been manufactured by forming a wet paper web on a fabric carrying sheet which then carries the paper web through a paper drying section to remove the excess water from the web. These paper drying sections have conventionally comprised rotatable steam-heated drum dryers over which the paper web traveled, so that the paper web was heated while travelling over the drum dryer to remove moisture from the paper web. The cost of supplying steam to these rotatable drum dryers for heating the dryers is quite high and the ability to maintain uniform drum surface temperatures and humidity in the dryers is difficult. Accordingly, dryer hoods are widely used in connection with these rotary drums, wherein pressurized drying air, instead of steam, is introduced at various points in the hood to contact one exposed surface of the wet web as it professes around the dryer with the exit path for the air being positioned on the other side of the rotary drum. This process is known as through air drying.
U.S. Pat. No. 3,432,936 issued to Sisson discloses one such drying assembly in which a moving stream of pressurized drying air is circulated about a paper web traveling about the periphery of a rotatable roll having apertures formed therein. Sisson utilizes a system where the hot drying air travels from the inside of the rotatable roll to the outside through the apertures, while the web travels about the outer surface of the roll. U.S. Pat. No. 3,432,936 issued to Cole et al. also discloses a drying assembly which utilizes through air drying by employing a configuration which moves drying air from the exterior of a rotatable roll through a paper web and into the interior of the rotatable roll, otherwise known as outside to inside drying air.
One of the most important shortcomings associated with the above-described paper drying machines utilizing through air drying is that the paper web is not uniformly dried throughout its surface due to the permeable carrying sheet which conveys the paper web through the drying procedure. As the drying air is forced through the permeable sheet, the drying air only passes through the permeable areas in the carrying sheet which, in turn, forces the air only through the portions of the paper web adjacent to the permeable areas in the carrying sheet. Therefore, the consistency of the paper web lacks uniformity due to the uneven drying which occurs pursuant to through air drying.
Other methods of drying a paper web have been utilized to more uniformly dry the web, such as passing the paper web under a series of infrared Imps as disclosed in U.S. Pat. No. 2,666,369 issued to Niks. Similarly, U.S. Pat. No. 3,639,207 issued to Genz et al. also discloses the use of infrared lamps to dry a paper web as it passes beneath the infrared lamps, wherein air is forced across the surface of the web to carry away stem that may be created from the heating of the wet paper web. However, no device associated with the prior art draws the hot air created from the infrared lamps through the paper web to increase drying efficiency.
In addition to the foregoing, with conventional paper drying and forming machines, vacuum pumps have been utilized to withdraw moisture from a paper web, such as U.S. Pat. No. 3,301,746 issued to Sanford et al. which discloses passing a wet paper web supported by a Fourdrinier wire over a series of suction boxes. However, these vacuum pumps merely draw the ambient air above the paper web through the web to withdraw moisture therefrom.
Therefore, as can be seen from the foregoing, there is clearly a pressing need for a paper drying machine which removes moisture from a paper web more efficiently as the web passes through a paper making device. In accordance with the present invention, this greater efficiency is accomplished by using an infrared drying device which heats the air about the web with this air being drawn through the paper web by a vacuum pump positioned on the opposite side of the paper web from the infrared drying device. Employing a paper drying apparatus as described above allows for a more efficient removal of moisture from a paper web, while also being compact in comparison to other known moisture removal methods.